Advanced lithographic techniques such as 193 nm immersion lithography have been developed to achieve high quality and smaller feature sizes in microlithography processes, for purposes of forming ever-smaller logic and memory transistors. It is important to achieve both smaller critical dimension (CD) in the imaged photoresist used in the microlithography process, and for the photoresists to provide both low line edge roughness (LER) and line width roughness (LWR), while still retaining good process control tolerances such as high exposure latitude (EL) and a wide depth of focus (DOE).
To meet the challenges for resist materials raised by high resolution lithography, photoacid generators (PAGs) with low, controllable diffusion properties are desirable, particularly ionic PAGs having a photoreactive cation and diffusion limiting anion. The structure of the PAG anion can affect the overall performance of a photoresist by affecting the interaction of the photoacid generator with other photoresist components. These interactions, in turn, affect the diffusion characteristics of the photogenerated acid. PAG structure and size can greatly affect the homogenous distribution of the PAG in the photoresist film. Defects such as T-topping, foot formation and notching can arise if the PAG is not uniformly distributed within the resist film.
Efforts have been made to limit diffusion and the attendant problems associated with diffusive acids. Photoacid generators in which a perfluoroalkysulfonate group is covalently linked to a single sterically bulky, aliphatic or heteroaliphatic group are known. For example, U.S. Pat. Nos. 7,301,047 B2 and 7,304,175 B2 disclose a bulky photoacid generator in which the sulfonate is linked to an adamantyl group. There remains a need for photoresist compositions including PAGs having controlled acid diffusion, improved miscibility with polymers and improved solubility in formulation solvents and organic developers.